perf(common): stop useTimeAgo re-rendering every second, add re-render profiler

measure-rerenders.mts counts React commits on an untouched board and attributes each
to the shallowest component that re-rendered, i.e. where the update entered the tree.

What it found: an idle board commits 73-96 times per minute, re-rendering
@mantine/core/Box over 30,000 times in 60s. That is the churn behind the 650 MB
browser peak — transient allocation rather than retention. About 80% of those commits
enter at the same anonymous depth-2 provider, props [value, children], value {appDir}:
Next.js's internal HeadManagerContext, whose value is a fresh object literal on every
App Router render, so each render re-renders the whole app beneath it. The rest enter
at __next_metadata_boundary__ / <title>, pointing the same way.

The trigger is inside Next's router/metadata handling — application code has no
polling router.refresh() — so nothing here fixes it. It is the next thing to chase and
it dominates everything downstream of it.

useTimeAgo called setState on a 1s interval even when the label was unchanged ("3
minutes ago" holds for a full minute), re-rendering its subtree ~60x/minute per
consumer for an identical string. It now compares before setting. Recorded as a
correctness fix and NOT a measured win: idle commits stayed inside their 73-96 range
afterwards, because the app-root churn dominates.

pnpm test 1794/1795 (pre-existing pi-hole Testcontainers flake); typecheck clean
across 37 packages; no new lint errors.

packages/common/src/hooks.ts

@@ -18,7 +18,14 @@ export const useTimeAgo = (timestamp: Date, updateFrequency = 1000) => {
   }, [timestamp]);
 
   useEffect(() => {
-    const intervalId = setInterval(() => setTimeAgo(calculateTimeAgo(timestamp)), updateFrequency);
+    // Only re-render when the rendered text actually changes. The label is coarse
+    // ("3 minutes ago" holds for a whole minute) but the timer ticks every second,
+    // so setting state unconditionally re-rendered the subtree ~60x/minute per
+    // consumer for an identical string.
+    const intervalId = setInterval(() => {
+      const next = calculateTimeAgo(timestamp);
+      setTimeAgo((previous) => (previous === next ? previous : next));
+    }, updateFrequency);
 
     return () => clearInterval(intervalId); // clear interval on hook unmount
   }, [timestamp, updateFrequency]);